1. Field of the Invention
This invention relates to the control of automated vehicles which move around a site, such as a factory or a warehouse, in accordance with control signals generated by a computer. The invention is particularly relevant to vehicles which are controlled over radio or other remote control links and do not rely on guidance wires or tracks.
2. Description of Related Art
British Patent No. 2,143,395 (assigned to the same assignee as the instant application) discloses such a system in which a number of mobile trucks are controlled and guided under the overall control of a base station. The trucks are utilised to transfer material between a store area and a work position. Finished work-pieces are transferred by means of one of the trucks to a holding area for removal and utilisation as required. The base station allocates destinations to each of the trucks via a communication link, such as a radio or infra-red link.
Each vehicle has a scanning laser beam which rotates in azimuth so that it scans across a number of reflector boards which are spaced apart around the site. Each reflector board is provided with uniquely coded strips of a retro-reflective material which is such that the laser beam incident thereon is reflected back along the same path. Each vehicle is therby able, using triangulation techiniques, to determine its own position relative to any location within the site. Each vehicle monitors its own position as it moves along a path to its required destination, and continuously transmits its position back to the base station, so that the base station can control the truck movements so as to avoid collisions.
The retro-reflective stripes and the non-reflective stripes therebetween form a unique bar code on each reflector board. When the stripes are scanned sequentially by the laser beam, the first few stripes in the sequence provide a code which confirms that a reflector board has been found (as distinct from any other reflective body which might be encountered). The next stripes in the sequence identify the particular target board which is being interrogated, and the final stripe indicates the position of the end of the reflector board with a high degree of accuracy, for determination of the position of the vehicle.
It will be apparent that the reflector boards must not be made too large, or they will encroach upon the area available for vehicle and personnel movements. On the other hand, the stripes must not be too narrow, or the reliability of code reading will be unacceptable. It follows, therefore, that, for an acceptable board size and an acceptable stripe width, only a limited number of stripes, and hence only a limited number of uniquely-coded boards, can be provided.
If a large site area is to be covered, it will not be possible to provide enough uniquely-coded boards to provide a workable system.